Almost all commercial polypropylenes are highly isotactic rather than atactic since highly isotactic polypropylenes are crystalline and provide structural functionality whereas atactic polypropylenes are amorphous and rubbery. In some cases poly(propylene oxide) is advantageous compared to polypropylene, e.g., in cases where adhering is important (since poly(propylene oxide) adheres better than polypropylene because of the oxygen in the backbone); however, all commercial poly(propylene oxide)s are atactic (and amorphous) and are not suitable for structural functionality and are not resistant to dissolving. On the other hand, isotactic poly(propylene oxide) is crystalline and suitable for structural functionality and is resistant to dissolving.
The reasons why commercial poly(propylene oxide)s are atactic is because of the high expense of optically active propylene oxide and because of the lack of catalysts that can isospecifically polymerize rac-propylene oxide, so that in almost all cases where isotactic poly(propylene oxide)s are synthesized, the product is a mixture of isotactic and atactic poly(propylene oxide)s. The mixture must be fractionated for recovery of isotactic poly(propylene oxide) e.g., by immersing the admixture in acetone below 0° C. which dissolves atactic poly(propylene oxide) but not isotactic poly(propylene oxide) and fractionation takes time and is expensive.
Catalysts have been developed that produce isotactic poly(propylene oxide) as a sole product (see Yoshino, N., et al., Macromol. Chem. 189, 1903-1913 (1988)), but said production requires a period of days and the m-dyad content is below 81%.
A higher dyad content has the advantage of a higher melting point and therefore suitability for higher temperature use.
Moreover, known catalysts that achieve high Mn provide high polydispersity index (PDI) and those that achieve low PDI provide low Mn.
As used herein PPO means poly(propylene oxide) and PO means propylene oxide.